Surfactants and hydrophilic colloid compositions and materials

ABSTRACT

Surfactants useful as dispersing aids in the preparation of compositions comprising a hydrophilic colloid having hydrophobic particles dispersed therein have the structure wherein M is a cation; X represents a group having the structure wherein each of R1, R2, R3 and R4 independently represents hydrogen or alkyl; or R1 and R2 taken together represent cycloalkyl; or R3 and R4 taken together represent cycloalkyl; and, n represents 0 or 1; provided that the total number of carbon atoms in each X group is 3 or 4, and when R1 is hydrogen R2 is other than methyl. Such surfactants offer coating and photographic property advantages when incorporated in multilayer photographic materials comprising a support bearing a plurality of hydrophilic colloid layers.

This is a Divisional of U.S. application Ser. No. 08/685,082, filed Jul.23, 1996, now U.S. Pat. No. 5,763,150.

FIELD OF THE INVENTION

The invention relates to surfactants and their use as dispersing aids inthe preparation of hydrophilic colloid compositions having hydrophobicparticles dispersed therein. Such compositions may be used in thepreparation of multilayer photographic materials.

BACKGROUND OF THE INVENTION

A wide variety of surfactants have been described for use in thepreparation of photographic materials.

JP56-19042 describes various diester sulfoitaconates as dispersing aidsfor photographic additives. The two ester linked hydrophobic groupsinclude a number of substituted or unsubstituted alkyl or aryl groups.

U.S. Pat. No. 3,948,663 describes photographic materials containingcertain sulfosuccinate surface active agents and refers to theirpossible use as dispersing aids and coating aids. A specific example ofsuch a surface active agent is sodium dioctyl sulfosuccinate which iscommercially available as Aerosol™OT.

WO93/03420 describes a method of making fine particle photographiccoupler dispersions which comprises forming a dispersion of photographiccoupler, coupler -solvent and auxiliary coupler solvent in an aqueousgelatin medium containing at least about 1% by weight of an anionicsurfactant having a hydrophobicity of 2-10 log P(OH) and washing thedispersion with water for a time sufficient to remove at least onefourth of the surfactant. Anionic surfactants of diverse structures maybe employed and included among several named surfactants isdiphenylbutyl sodium sulfosuccinate.

A shortcoming of the use of surfactants described in JP56-19042 and U.S.Pat. No. 3,948,663 is the very low surface tension values exhibited bythe compounds at concentrations above their critical micelleconcentration (CMC). In the simultaneous multilayer coating ofhydrophilic colloid layers, it is essential that the surface tension ofthe top layer is lower than that of any of the underlying layers if itis to remain spread during the coating operation. If one of theunderlying layers has a lower surface tension than the top layer itdrives the top layer in from the edges towards the centre of thecoating. This is often termed "edge retraction". The larger the surfacetension imbalance, the more disruptive is the effect. Large differencescan cause retraction of the whole coating pack and general layerinversions. The surface tension of underlying layers in the multilayercoating of photographic materials is often dominated by the surfactantdispersing aid that is used to stabilize the emulsified hydrophobicparticles therein e.g. color couplers and their associated solvents.

When such prior art surfactants are used as dispersing aids foremulsified materials that are incorporated in underlying hydrophiliccolloid layers during simultaneous multilayer coating, a constraint isput on the choice of surfactant or surfactant concentration required forthe overlying layers i.e. coating latitude is relatively narrow.

Another shortcoming of the use of the surfactants described inJP56-19042 and U.S. Pat. No. 3,948,663 as dispersing aids forphotographic couplers in hydrophilic colloid compositions is that thephotographic properties of such compositions e.g. the liquid dispersionreactivity, can be less than desired.

PROBLEM TO BE SOLVED BY THE INVENTION

The invention overcomes the coating latitude problem associated withsome of the prior art dipersing aids.

Limitations in the photographic properties of dispersions ofphotographic couplers in hydrophilic colloids can be overcome.

SUMMARY OF THE INVENTION

The invention provides compounds having the structure ##STR3## wherein Mis a cation;

X represents a group having the structure ##STR4## wherein each of R¹,R², R³ and R⁴ independently represents. hydrogen or alkyl; or R¹ and R²taken together represent cycloalkyl; or R³ and R⁴ taken togetherrepresent cycloalkyl; and,

n represents 0 or 1;

provided that the total number of carbon atoms in each X group is 3 or4, and when R¹ is hydrogen R² is other than methyl.

The invention also provides a composition comprising a hydrophiliccolloid having hydrophobic particles dispersed therein with the aid of asurfactant having the structure I.

A multilayer photographic material comprises a support bearing aplurality of hydrophilic colloid layers including at least onelight-sensitive silver halide emulsion layer wherein at least one of theunderlying layers of the material contains hydrophobic particlesdispersed therein with the aid of a surfactant having the structure I.

A method of preparing a multilayer photographic material comprises

(a) simultaneously coating on a support a plurality of aqueoushydrophilic colloid layers including at least one light-sensitive silverhalide emulsion layer wherein at least one of the underlying layerscontains hydrophobic particles dispersed therein with the aid of asurfactant having the structure I, and

(b) drying the coated layers.

ADVANTAGEOUS EFFECT OF THE INVENTION

By providing aqueous hydrophilic colloid melts with high surface tensionminima, the invention enables increased coating latitude.

Improved photographic performance can be achieved with dispersions of aphotographic coupler in a hydrophilic colloid. The nature of theimprovement depends on the type of coupler dispersion. For example, witha microprecipitated dispersion, the benefits include increased liquiddispersion reactivity. With a homogenized dispersion, the benefitsinclude increased contrast in coated product.

DETAILED DESCRIPTION OF THE INVENTION

In structure I, the cation M is a positively charged atom or group ofatoms preferably chosen from alkali metal cations e.g. Na⁺ ; ammonium ortetraalkylammonium.

R¹, R², R³ and R⁴ may be selected from the group consisting of methyl,ethyl and propyl; R¹ and R² taken together or R³ and R⁴ taken togethermay be selected from the group consisting of cyclopropylene andcyclobutylene; provided that the total number of carbon atoms in each Xgroup is 3 or 4, and when R¹ is hydrogen R² is other than methyl.

Preferred compounds include those wherein R¹ and R² are eachindependently alkyl, or R³ and R⁴ are each independently alkyl.

A particularly preferred compound is represented by structure I whereinn is 1, R¹ and R² each represent hydrogen and R³ and R⁴ each representmethyl.

The compounds may be water soluble or water dispersible.

The compounds may be prepared by the esterification of maleic acid withan appropriate phenylalkanol. A specific method which can be used inrespect of all the compounds is given below in Example 1.

Compositions comprising a hydrophilic colloid having hydrophobicparticles dispersed therein may be formed by a process comprisingdispersing a hydrophobic material into an aqueous solution of ahydrophilic colloid in the presence of the surface active agent.

For homogenized dispersions, the surface active agent is used preferablyin an amount from 0.4 to 2.0, more preferably from 0.6 to 0.9 weightpercent based on the weight of the aqueous dispersion.

For microprecipitated dispersions, the surface active agent is usedpreferably in an amount that provides a molar ratio of surface activeagent: hydrophobic material e.g. photographic coupler which is from 1:4to 2:1.

Regardless of the particular method of preparation, dispersions can bemade in accordance with the invention which avoid the coating latitudeproblems associated with the prior art by using less than about 1 weightpercent of the surfactant and without requiring a washing step to removeat least one fourth of the surfactant.

The invention is particularly useful in the preparation of photographiccompositions and materials.

In the following discussion of suitable materials for use in thecompositions and materials of this invention, reference will be made toResearch Disclosure, December, 1989, Item 308119, published by KennethMason Publications, Ltd., Dudley Annex, 12a North Street, Emsworth,Hampshire, P010 7DQ, UK. This publication will be identified hereafterby the term Research Disclosure.

A number of hydrophobic photographic additives used in light sensitivephotographic materials are oil-soluble and are used by dissolving themin a substantially water-insoluble, high boiling point solvent which isthen dispersed in an aqueous hydrophilic colloid solution with theassistance of a dispersing aid. Such oil-soluble additives include imageforming dye couplers, dye stabilizers, antioxidants and ultra-violetradiation absorbing agents. A typical solvent used to dissolve theadditive is aromatic e.g. di-n-butyl phthalate.

Gelatin is the preferred hydrophilic colloid, but other hydrophiliccolloids can be used alone or in combination with gelatin.

Suitable methods of preparing photographic dispersions are described inResearch Disclosure, Sections XIV A and XIV B. For example, homogenisedoil in aqueous gelatin dispersions of photographic couplers can beprepared by dissolving the coupler in a coupler solvent and mechanicallydispersing the resulting solution in an aqueous gelatin solution (seeU.S. Pat. No. 2,322,027).

Alternatively, microprecipitated dispersions of photographic couplersprepared by solvent and/or pH shift techniques are becoming more widelyused (see references: U.K. Patent No. 1,193,349; Research Disclosure16468, December 1977 pp 75-80; U.S. Ser. No. 288,922 (1988) by K. Chari;U.S. Pat. Nos. 4,970,139 & 5,089,380 by P. Bagchi; U.S. Pat. No.5,008,179 by K.Chari, W. A. Bowman & B. Thomas; U.S. Pat. No. 5,104,776by P. Bagchi & S. J. Sargeant) and offer benefits in decreased dropletsize and often increased reactivity relative to conventionaloil-in-water homogenised dispersions.

Multilayer photographic materials according to the invention compriseone or more underlying layers formed from such compositions.

Preferred multilayer photographic materials include color materials ofthe type described in Research Disclosure, Sections VII A to VII K.

Methods of preparing multilayer photographic materials by simultaneouslycoating the layers are known. Particular methods are described inResearch Disclosure. Sections XV A and XV B. Such methods includeextrusion coating and curtain coating.

The hydrophobic material dispersed in the hydrophilic colloid may be aphotographic coupler.

Couplers which form cyan dyes upon reaction with oxidizedcolor-developing agents are described in such representative patents andpublications as U.S. Pat. Nos. 2,772,162; 2,895,826; 3,002,836;3,034,892; 2,747,293; 2,423,730; 2,367,531; 3,041,236; and 4,333,999;and Research Disclosure, Section VII D.

Couplers which form magenta dyes upon reaction with oxidized colordeveloping agents are described in such representative patents andpublications as: U.S. Pat. Nos. 2,600,788; 2,369,489; 2,343,703;2,311,082; 3,152,896; 3,519,429; 3,062,653; and 2,908,573; and ResearchDisclosure, Section VII D.

Couplers which form yellow dyes upon reaction with oxidized and colordeveloping agents are described in such representative patents andpublications as: U.S. Pat. Nos. 2,875,057; 2,407,210; 3,265,506;2,298,443; 3,048,194; and 3,447,928;, and Research Disclosures, SectionVII D.

Couplers which form colorless products upon reaction with oxidized colordeveloping agents are described in such representative patents as: UKPat. No. 861,138; U.S. Pat. Nos. 3,632,345; 3,928,041; 3,958,993; and3,961,959.

The couplers can be dissolved in a solvent and then dispersed in ahydrophilic colloid. Examples of solvents usable for this processinclude organic solvents having a high boiling point, such as alkylesters of phthalic acid (for example, dibutyl phthalate, dioctylphthalate, and the like), phosphoric acid esters (for example, diphenylphosphate, triphenyl phosphate, tricresyl phosphate, dioctyl butylphosphate, and the like) citric acid esters (for example, tributylacetyl citrate, and the like) benzoic acid esters (for example, octylbenzoate, and the like), alkylamides (for example, diethyl laurylamides,and the like), esters of fatty acids (for example dibutoxyethylsuccinate, dioctyl azelate, and the like), trimesic acid esters (forexample, tributyl trimesate, and the like), or the like; and organicsolvents having a boiling point of from about 30° to about 150° C., suchas lower alkyl acetates (for example, ethyl acetate, butyl acetate, andthe like), ethyl propionate, secondary butyl alcohol, methyl isobutylketone, b-ethoxyethyl acetate, methyl cellosolve acetate, or the like.Mixtures of organic solvents having a high boiling point and organicsolvents having a low boiling point can also be used.

As the binder or the protective colloid for the photographic emulsionlayers or intermediate layers of the photographic light-sensitivematerial of the present invention, gelatin is advantageously used, butother hydrophilic colloids can be used alone or together with gelatin.

As gelatin in the present invention, not only lime-processed gelatin,but also acid-processed gelatin may be employed. The methods forpreparation of gelatin are described in greater detail in Ather Veis,The Macromolecular Chemistry of Gelatin, Academic Press (1964).

As the above-described hydrophilic colloids other than gelatin, it ispossible to use proteins such as gelatin derivatives, graft polymers ofgelatin and other polymers, albumin, casein, and the like; saccharidessuch as cellulose derivatives such as like, sodium alginate, starchderivatives, and the like; and various synthetic hydrophilic highmolecular weight substances such as homopolymers or copolymers, forexample, polyvinyl alcohol, polyvinyl alcohol semiacetal,poly-N-vinylpyrrolidone, polyacrylic acid, polymethacrylic acid,polyacrylamide, polyvinyl imidazole, polyvinylpyrazole, and the like.

In the photographic emulsion layers or other hydrophilic colloid layersof the photographic light-sensitive material of the present inventioncan be incorporated various surface active agents as coating aids or forother various purposes, for example, prevention of charging, improvementof slipping properties, acceleration of emulsification and dispersion,prevention of adhesion and improvement of photographic characteristics(for example, development acceleration, high contrast,and.sensitization), and the like.

Surface active agents which can be used are nonionic surface activeagents, for example, saponin (steroid-based), alkyene oxide derivatives(for example, polyethylene glycol, a polyethylene glycol/polypropyleneglycol condensate, polyethylene glycol alkyl ethers or polyethyleneglycol alkylaryl ethers, polyethylene glycol esters, polyethylene glycolsorbitan esters, polyalkylene glycol alkylamines or polyalkylene glycolalkylamides, and silicone/polyethylene oxide adducts, and the like),glycidol derivatives (for example, alkenylsuccinic acid polyglycerideand alkylphenol polyglyceride, and the like), fatty acid esters ofpolyhydric alcohols and alkyl esters of sugar, and the like; anionicsurface active agents containing an acidic group, such as a carboxygroup, a sulfo group, a phospho group, a sulfuric acid esters group, anda phosphoric acid ester group, for example, alkylcarboxylic acid salts,alkylsulfonic acid salts, alkylbenzenesulfonic acid salts,alkylnaphthalenesulfonic acid salts, alkylsulfuric acid esters,alkylphosphoric acid esters, N-acyl-N-alkyltaurines, sulfosuccinic acidesters, sulfoalkylpolyoxyethylene alkylphenyl ethers, andpolyoxyethylene alkylphosphoric acid esters; amphoteric surface activeagents, such as amino acids, aminoalkylsulfonic acids,aminoalkylsulfuric acid or aminoalkylphosphoric acid esters,alkylbetaines, and amine oxides; and cationic surface active agents, forexample, alkylamine salts, aliphatic or aromatic quaternary ammoniumsalts, heterocyclic quaternary ammonium salts (for example, pyridiniumand imidazolium) and aliphatic or heterocyclic phosphonium or sulfoniumsalts.

The surfactant dispersing aid of this invention may be used inconjunction with other surfactants, e.g. anionic and/or nonionicsurfactants, which may be present for auxiliary purposes such as,additional dispersing aid, improved coating uniformity and/or rheologymodification (e.g. reduced viscosity and shear thinning).

In the photographic emulsion layer of the photographic light-sensitivematerial used in the present invention, any of silver bromide, silveriodobromide, silver iodochlorobromide, silver chlorobromide and silverchloride may be used as the silver halide.

The light-sensitive silver halide contained in the photographic materialcan be processed following exposure to form a visible image byassociating the silver halide with an aqueous alkaline medium in thepresence of a developing agent contained in the medium or the material.Suitable types of photographic processing are described in ResearchDisclosures, Section XIX A to XIX J. Suitable developing agents aredescribed in Research Disclosures, Section XX A to XX B.

The following Examples further illustrate the invention.

A number of compounds referred to in the Examples are as follows:

    ______________________________________    Structure or    Commercial Name or          Code    publication source                   Chemical Name                                (where used)    ______________________________________    C.sub.12 H.sub.25.OSO.sub.3.Na                   Sodium dodecyl                                SDS                   sulphate    1 #STR5##      Sodium dodecylbenzene sulphonate                                SDBS    Aerosol ™ OT                   Sodium di-2- AOT    (Cyanamid)     ethylhexyl                   sulphosuccinate    X = --(CH.sub.2).sub.3 --                   Sodium Di-4- X = --(CH.sub.2).sub.3 --    (structure I)  phenylbutyl  (structure I)                   sulphosuccinate    X = --(CH.sub.2).sub.2 --                   Sodium Di-3- X = --(CH.sub.2).sub.2 --    (structure I)  phenylpropyl (structure I)                   sulphosuccinate    ______________________________________

EXAMPLE 1 Synthesis of sodium di(2,2-dimethyl-3-phenylprop-1-yl)sulphosuccinate

A mixture of maleic acid (116.0 g, 1.00 mol), and2,2-dimethyl-3-phenyl-1-propanol (328.5 g, 2.00 mol) and concentratedsulphuric acid (0.5 cm³) was suspended in toluene (260 cm³) and refluxedfor 5 hours in a flask equipped with a Dean and Stark trap. On cooling,the toluene solution was diluted with ethyl acetate (800 ml) and washedwith 1M sodium hydrogen carbonate (2×500 cm3). The organic layer wasdried over magnesium sulphate and the solvent removed under reducedpressure (15 mm Hg, 50° C.) to give an intermediate diester as a clearoil (363.3 g, 89%).

A solution of sodium metabisulphite (104.5 g, 0.55 mol) in water (350cm³) was added to a solution of this diester (204.2 g, 0.50 mol) inethanol (350 cm³) and the mixture brought to reflux over 15 minutes.Sodium sulphite (56.7 g, 0.45 mol) was then added portionwise to themixture over 30 minutes and the reaction refluxed for 5.5 hours. Thereaction mixture was evaporated at reduced pressure (15 mm Hg, 22° C.)to remove ethanol and the resulting aqueous solution was then extractedinto ethyl acetate (2×400 cm³). The organic solution was dried overanhydrous magnesium sulphate, filtered, evaporated at reduced pressure(15 mm Hg, 22° C.), and finally freeze dried to give the product as awhite crystalline solid (203.2 g, 79%). Data from infra-red and 1H NMRspectroscopy was consistent with the proposed product, sodiumdi(2,2-dimethyl-3-phenylprop-1-yl)sulphosuccinate.

EXAMPLE 2 Surface Tension Measurements

Surface tension measurements were conducted in solutions containing 7%deionised Type IV bone gelatin in water at 40° C. to simulate a coatingmelt. Static surface tensions were measured as a function of surfactantconcentration using the wilhelmy technique.

The measurement of surface tension of an aqueous solution containingsurfactant was measured over a concentration range including thecritical micelle concentration using the Wilhelmy technique Padday J F.2nd Int. Congress of Surface Activity, I, 1, 1957! with a platinumblade.

Table 1A compares static surface tension data of a compound of theinvention with comparison compounds.

                  TABLE 1A    ______________________________________    Static Surface Tension (mN/m) of Solutions    in 7% Deionised Type IV Bone Gelatin Water at 40° C.                   Wt % Concentration in 7% Deionised    Compounds Tested                   Gelatin solution    X              0.10%       0.30%    ______________________________________    Invention      36.7        36.3    X = --CH.sub.2 C(CH.sub.3).sub.2 --    Comparison     42.8        42.8    X = --(CH.sub.2).sub.3 --    Comparison     28.9        28.7    AOT    ______________________________________

Comparative dynamic surface tension measurements were also determined bythe same technique using an overflowing circular weir ibid.!. Theaverage surface age of the solutions in the overflowing weir has beenestimated to be of the order of 0.1 seconds.

The particular conditions of use for the dynamic surface tensionmeasurements were:

(i) Diameter of lip of circular weir, 37.5 mm;

(ii) No sintered glass disc;

(iii) Flow rate over weir, 9 ml/sec.

(iv) Temperature, 40° C. for aqueous gelatin solutions. Dynamic surfacetensions (DST) of this order of surface age have been found to be morerelevant to the process of coating multilayers of photographic elementsthan static/equilibrium values. Table 1B shows the corresponding DSTdata measured using the `Weir` technique.

                  TABLE 1B    ______________________________________    Dynamic Surface Tension (mN/m) of Solutions    in 7% Deionised Type IV Bone Gelatin Water at 40° C.    using the Weir Method                   Wt % Concentration in 7% Deionised    Compounds Tested                   Gelatin solution    X              0.10%       0.30%    ______________________________________    Invention      37.6        36.3    X = --CH.sub.2 C(CH.sub.3).sub.2 --    Comparison     43.7        42.8    X = --(CH.sub.2).sub.3 --    Comparison     30.1        29.2    AOT    ______________________________________

At low concentrations DST values are higher than the correspondingstatic surface tension values. However, as is seen here, the DST valuesapproach the limiting static values at fairly low concentration levelsif surfactants possess a moderate balance of hydrophilic and lipophilicproperties.

The di-ω-phenylalkyl sulphosuccinate of this invention gives much highervalues of static and dynamic surface tension than corresponding dialkylsulphosuccinates such as Aerosol OT. Hence the compounds of the currentinvention maintain the advantage over aliphatic sulphosuccinates of ahigher limiting surface tension value, and as such can be expected togive a better coating latitude.

EXAMPLE 3 Increased Dispersion Reactivity with MicroprecipitatedDispersions

Microprecipitated dispersions were made with the Coupler A shown below##STR6## and various dispersing aids.

The Coupler A (20 g) was dissolved in a mixture of propan-1-ol (40 g)and 20% sodium hydroxide solution (5 g) at 60° C. and poured into asolution of surfactant (weight equimolar with coupler) andpolyvinylpyrrolidone (10 g) in water (600 g). The resulting micellarsolution was reduced to pH 6.0 by the dropwise addition of 15% propanoicacid, to form the crude microprecipitated dispersion which was thendialysed through Amicon hollow fibre ultrafiltration cartridges andconcentrated to a fifth of its volume.

The liquid dispersion reactivity measurements were made according to themethod described by Bagchi in U.S. Pat. Nos. 4,970,139; 5,089,380 and5,104,776. Particle size was measured by photon correlationspectroscopy. The objective of the study was to compare the performanceof the surfactant of the invention as a dispersing aid against othersurfactants.

                  TABLE 2    ______________________________________    Microprecipitated dispersions of Coupler A    Physical data.                Liquid    SURFACTANT  Dispersion                          Mean Particle    (dispersing aid)                Reactivity                          Diameter (nm)                                      Comment    ______________________________________    X = --CH.sub.2 C(CH.sub.3).sub.2 --                6597      152         invention    X = --(CH.sub.2).sub.3 --                5891      88          comparison    AOT         5188      192         comparison    SDBS        5386      219         comparison    SDS         4750      39          comparison    ______________________________________

The above results show that the compound of this invention increases theliquid dispersion reactivity of the microprecipitated dispersionrelative to other surfactants. In particular, the invention shows asignificant advantage in reactivity over the similarly structureddi-4-phenylbutyl sulphosuccinate (ratio of reactivities=1.12). Asdemonstrated here, the prior art compounds give higher reactivity thanthe aliphatic sulphosuccinate, AOT, and other conventional anionicsurfactants.

Interestingly, the dispersion reactivity obtained in Table 2 issurfactant specific and does not correlate with the measured particlesize.

EXAMPLE 4 Increased Contrast and Dmax with Homogenised Dispersions ofColour Coupler A

Traditionally (see U.S. Pat. No. 2,322,027 by Jelly and Vittum) colorcouplers are dissolved in a high-boiling, water-insoluble solvent andmechanically dispersed in an aqueous gelatin solution containingsurfactant to facilitate dispersion. Mean droplet sizes are usuallysignificantly larger (typically, 0.2 μm) than those produced bymicroprecipitation techniques (typically, 0.02 μm).

The homogenised dispersions were made according to the followingtechnique.

A dispersion was made of the following general formula:

Coupler A 11.7%

di-n-butylphthalate 3.9%

gelatin 9.5%

water & surfactant 74.8%

Coupler A was dissolved in di-n-butyl phthalate and heated at 140° C.until the coupler had completely dissolved. Gelatin was dissolved inwater and heated to 70° C. Surfactant was added to the gelatin solutionat a rate of 0.1 mole equivalent to coupler. The coupler solution wasthen added to the gelatin solution and homogenised for 3 minutes using aKinematica Polytron set at 10,000 rpm and then passed (twice) through aMicrofluidics Microfluidiser (model no. 110E) which was run at 10,000psi pressure and a water bath temperature of 75° C.

A monochrome bilayer format was used for the photographic evaluation ofthe coupler dispersions. The two layers were coated simultaneously overa "gel" pad coated support as illustrated in the diagram below:

    ______________________________________    TOP    Layer 2   Gelatin         1.614 g/m2              Alkanol XC      21.5 mg/m2              Hardener BVSME* Added at rate of 1.8%                              w/w of total gelatin,                              including "gel" pad    Layer 1   Gelatin         1.614 g/m2              Coupler A       0.836 mmoles/m2              Silver              (as chloride emulsion)                              239.0 mg/m2    "Gel" Pad Gelatin         3.229 g/m2    Support/////////////                    Resin-coated paper ////////////    ______________________________________     *BVSME: Bisvinyl-sulphonyl-methyl-ether

The coatings were exposed to white light for 0.1s through a 21 step 0.15logE increment tablet and processed in standard RA-4 chemistry. The Dmaxdensity and contrast were measured using an analytical reflectiondensitometer. Results are shown in Table 3 below.

                  TABLE 3    ______________________________________    Measurements of coated dispersions of    Coupler A made with different surfactants.    SURFACTANT     Contrast                           Dmax    (dispersing aid)                   (±0.06)                           (±0.2)  Comment    ______________________________________    X = --CH.sub.2 C(CH.sub.3).sub.2 --                   4.90    2.79       invention    X = --(CH.sub.2).sub.3 --,                   4.76    2.67       comparison    X = --(CH.sub.2).sub.2 --                   4.79    2.60       comparison    AOT            4.50    2.53       comparison    SDBS           4.63    2.59       comparison    ______________________________________

Table 3 shows that the dispersing aid of this invention gives improvedcontrast and Dmax relative to the three types of dispersing aid:

(i) The relative straight chain di-ω-phenylalkyl sulphosuccinates.

(ii) A typical aliphatic sulphosuccinate, such as Aerosol™ OT.

(iii) A typical conventional anionic surfactant, such as SDBS.

We claim:
 1. A compound having the structure ##STR7## wherein M is acation;X represents a group having the structure ##STR8## wherein n is1, R¹ and R² each represent hydrogen and R³ and R⁴ each representmethyl.